In cable television signal distribution systems, many separate RF television channels are simultaneously transmitted over a transmission line, and a required bandwidth thereof may extend from as low as about 40 MHz to as high as about 1 GHz. Presently, cable television distribution systems conventionally employ coaxial cable as the transmission line for the RF spectra. Due to the significant signal losses inherent with coaxial cable transmission lines, repeater amplifiers must usually be provided at closely spaced distances, such as at every half mile along the extent of the network. These repeaters are expensive, require a constant power supply, introduce cumulative distortion, and must be maintained in order to provide continuous cable service to subscribers.
Analog fiber optic networks have been proposed as a replacement for coaxial cable transmission lines within broadband telecommunications and cable television networks. Such networks operate successfully over vastly greater distances between repeaters. Typically, a laser diode may be used as a transmitter within such networks. Unfortunately, laser diodes are known to produce intermodulation distortion. Modern distributed feedback ("DFB") lasers exhibit significant and unacceptable amounts of intermodulation distortion, particularly composite second order (CSO) distortion products, i.e. distortion products of the type 2f.sub.1, 2f.sub.2, f.sub.2 -f.sub.1, and f.sub.2 +f.sub.1. Though second order intermodulation distortion can be eliminated by limiting an RF signal to a narrowband range, i.e. by keeping a maximum frequency f.sub.mx less than twice a value of a minimum frequency f.sub.mn, this solution is relatively expensive since higher transmission frequencies are required for a given bandwidth. Predistortion techniques have therefore been proposed as one method for improving distortion levels at the DFB laser output for broadband transmissions. As used herein "broadband" means any signal where the highest frequency being transmitted is greater than twice the lowest frequency being transmitted.
"Low cost" DFB lasers designed for analog applications exhibit CSO levels in the 52-56dB range below a level of the fundamental at five percent optical modulation index (OMI) for a 36 channel phase alternating line (PAL) frequency plan. In order to meet a nominal 67dB CSO level below the fundamental, a predistortion circuit must yield a minimum of 15dB improvement, and be stable over time and temperature.
Blauvelt et al., U.S. Pat. No. 4,992,754, the disclosure of which is incorporated herein by reference, discloses a predistortion circuit whereby an RF input signal is split into a primary and secondary electrical path, with composite second order distortion products being generated in the secondary electrical path by two amplifiers arranged in a push-push configuration. The distorted electrical signal in the secondary path is then combined into the primary electrical path and fed to the laser. A disadvantage of this technique is that the push-push amplifier configuration requires a 180.degree. hybrid splitter. Such splitters inherently have significant nonlinear frequency characteristics that introduce amplitude and phase errors that can limit the cancellation levels achieved by the predistortion circuit
Single channel architectures utilizing transistors (FETs) and diodes have also been proposed. However, these designs fail to achieve acceptable levels of distortion reduction over a multichannel broadband frequency. Also, these approaches are generally not capable of dealing effectively with lasers having frequency dependent distortion characteristics and therefore do not achieve more than 3-5dB of improvement.